Issue #20048: Fixed ZipExtFile.peek() when it is called on the boundary of

the uncompress buffer and read() goes through more than one readbuffer.

This is partial backport of changeset 028e8e0b03e8.

Lib/zipfile.py

@@ -606,7 +606,11 @@ class ZipExtFile(io.BufferedIOBase):
         """Returns buffered bytes without advancing the position."""
         if n > len(self._readbuffer) - self._offset:
             chunk = self.read(n)
-            self._offset -= len(chunk)
+            if len(chunk) > self._offset:
+                self._readbuffer = chunk + self._readbuffer[self._offset:]
+                self._offset = 0
+            else:
+                self._offset -= len(chunk)
 
         # Return up to 512 bytes to reduce allocation overhead for tight loops.
         return self._readbuffer[self._offset: self._offset + 512]

Misc/NEWS

@@ -27,6 +27,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20048: Fixed ZipExtFile.peek() when it is called on the boundary of
+  the uncompress buffer and read() goes through more than one readbuffer.
+
 - Issue #20034: Updated alias mapping to most recent locale.alias file
   from X.org distribution using makelocalealias.py.